It is well known that the frequency spectrum of a radio transmission between a moving vehicle (mobile body) and a base station fixed to the ground undergoes some variations coming from the Doppler effect. Such an effect corresponds to a phase noise that the electromagnetic waves from such radio link are subjected to when the direction of motion of the mobile body is parallel to the direction of the propagating electromagnetic wave. This phase noise or signal fading can be usually neglected for actually used radio transmission techniques based on frequency channels well separated (see GSM or even UMTS).
But this situation may no more be the case for techniques based on multicarrier transmission trying to use its full capacity. This is typically the case for e.g. Orthogonal Frequency Division Multiplexing (OFDM), the technique used for radio transmission compatible to the IEEE standard 802.16 also known under the acronym Worldwide Interoperability for Microwave Access WIMAX which seems today one of the most promising technology under discussion for bidirectional communication to mobiles. For such technologies, the sensitivity to the Doppler effect may no more be so negligible since the frequency channels are chosen with a very narrow frequency spacing. For example, at OFDM the frequency spacing is arranged so as to null the correlation between a modulation band signal transmitted by a nth subcarrier of multicarrier transmission and a modulation band signal transmitted by a (n+1)th subcarrier.
In accordance with OFDM, the frequency assignment with overlapping bands becomes possible, thereby enabling an improvement in the spectrum efficiency. OFDM is different from other multicarrier transmission schemes that modulate theirs carriers independently, and since modulation/demodulation is performed at a stroke by a Fast Fourier Transform (FFT), an orthogonal relationship is established among the carriers. Further, by adding on a guard interval signal on the transmitting side, it is possible to eliminate inter-symbol interference caused by multipath delay. If an IFFT output signal conforming to one OFDM symbol is adopted as one unit, insertion of the guard interval signifies copying the tail-end portion of the signal to the leading end thereof. Thus, with OFDM, multipath equalization basically is unnecessary. However, in order to avoid causing a decline in performance, a guard interval that is larger than the maximum delay time of multipath envisioned in the system must be set in such a manner that inter-symbol interference will not occur. Though inserting the guard interval makes it possible to eliminate the influence of interference caused by multipath, a tradeoff is involved in that the guard interval diminishes transmission efficiency at the same time. In order to mitigate the decline in transmission efficiency, it is necessary to make the OFDM symbol duration as large as possible, i.e. to make the guard ratio as small as possible. From this viewpoint, the carrier spacing in the given bandwidth should be made small, i.e., the number of carriers should be increased.
However, due to fading, the received signal varies not only along the time direction but also along the frequency direction, latter one being the Doppler shift. Doppler shift which is directly proportional to the speed of motion of the mobile body is produced in the range of maximum Doppler frequency. If the carrier spacing is small, this variation is greater than one carrier and carrier synchronization on the receiving side is difficult. As a consequence, frequency-selective fading, in which the variation sustained differs depending upon the frequency, occurs and the performance at the receiver is degraded. The reason for this is that inter-carrier interference occurs because frequency fluctuation is independent from carrier to carrier (or more specifically, from carrier group to carrier group within the coherence bandwidth). In order to suppress the degradation of performance caused by that interference, it is necessary to make the carrier spacing as large as possible. Thus, there is a tradeoff with regard to transmission efficiency.
In EP1 460 780 B1 is described an antenna apparatus capable of being installed at a mobile body, the antenna apparatus comprising a plurality of receiving antenna. These antenna are controlled by an antenna switches for switching each of the plurality of receiving antennas between a connected and a disconnected state respectively. An information processing circuit controls that switches based on direction and speed at which the vehicle moves relative to direction of propagation of the received signal. This information is determined from the known position of the broadcast station and the current vehicle position derived from e.g. GPS. It allows to inhibit Doppler effect when a vehicle receives an OFDM signal, and hence allows good reception even when the vehicle is moving by controlling the switching of antennas based on likely occurrence of Doppler effects rather than signal level, which is not necessarily different between antennas.
For IEEE 802.16 standard which is today one of the favorite technologies under discussion for bidirectional communication to mobiles (mobile body) the following estimations apply: up to 100 km/h negligible influence; up to 200 km/h slight degradation of signal quality, but can still be tolerated; beyond 200 km/h the effect becomes more and more important and decreases signal quality significantly. At 400 km/h which is the target speed for modern long distance trains, a Doppler compensation is absolutely required. Such a picture may be worse i.e. the speed limit beyond which the Doppler effect becomes more and more important may be less when the used technology is based on a smaller frequency channel spacing to increase transmission capacity.